1. Field of the Invention
The present invention relates to an optical module having a semiconductor light-emitting device or a semiconductor light-detecting device and a lens which are coaxially aligned with each other in a housing of synthetic resin, and more particularly to an optical module having a spherical lens held in position by a plurality of teeth integrally formed with and disposed in a housing of synthetic resin.
2. Description of the Related Art
Optical modules have a semiconductor device, which may be a semiconductor light-emitting device such as a semiconductor laser or a semiconductor light-detecting device such as a photodiode, and a lens which are coaxially aligned with each other. The optical modules are used in the field of optical communications. In computer systems for data communications, for example, an optical module having a semiconductor light-emitting device and an optical module having a semiconductor light-detecting device are paired on a board. Each of such optical modules comprises a semiconductor device, a lens, and a housing which holds the semiconductor device and the lens and includes a structure for fittingly holding the ferrule of an optical plug to be connected thereto. When the optical plug is connected to the optical module, the semiconductor device is optically coupled to the optical fiber in the ferrule by the lens. The housing generally comprises a holder for holding the semiconductor device and the lens and a receptacle for fittingly holding the ferrule of the optical plug. The holder and the receptacle are separately manufactured, and subsequently secured to each other for coaxial alignment.
Lenses incorporated in optical modules are mostly spherical lenses because they can easily and inexpensively be manufactured to high accuracy only by mechanical machining processes and they can easily be assembled in place without any orientational adjustment as they have no directionality. A spherical lens may be fixed to a holder by a bonding process or a fusion process. According to the bonding process, the lens is placed into and positioned in a recess in the holder, and thereafter a resin adhesive is applied to a surrounding surface of the lens and then thermoset. According to the fusion process, after the lens is placed into and positioned in the recess in the holder, a ring-shaped body made of glass having a low melting point, which is slightly greater than the outside diameter of the lens, is inserted around the lens and then melted with heat.
The bonding process presents a handling problem before the resin adhesive is thermoset because the resin adhesive in a liquid state is poured into a small region around the lens. Another problem is that since the holder is usually made of metal, the bonded surface of the lens tends to thermally crack due to the difference between the coefficients of thermal expansion of the lens and the holder.
The fusion process is also disadvantageous in that the lens suffers devitrification when left at high temperature and high humidity, the bonding strength is poor, and the cost is high. The ring-shaped body made of glass having a low melting point is produced by pressing a powder of low-melting-point glass to shape. Therefore, when the ring-shaped body is placed into position around the lens, fine fragments or powder particles from the ring-shaped body are apt to be scattered and attached to the lens surface. If the ring-shaped body is melted with heat to secure the lens in position, the fine fragments or powder particles attached to the lens surface are fused, and the glass of the ring-shaped body flows around, producing a local film of low-melting-point glass on the lens surface. Inasmuch as the low-melting-point glass is not highly resistant to humidity, it is often subject to devitrification with time, and the devitrified glass is responsible for a reduction in the intensity of light passing through the lens. The surface of the low-melting-point glass around the lens is liable to be fragile due to minute cracks that have been developed therein by humidity, thus lowering the strength with which the lens is secured to the point where the lens may possibly fall off the holder. A separate humidity resistance scheme is necessary to eliminate these shortcomings. This, together with the fact that the ring-shaped body of low-melting-point glass is expensive, leads to an increase in the overall cost of the optical module.
The inventor of the present invention has proposed an optical module comprising a lens holder for securely holding a lens as disclosed in the U.S. patent application Ser. No. 09/094,774. The lens holder of the disclosed optical module comprises a plurality of teeth and a lens seat surface which are integrally formed with a housing of synthetic resin. The disclosed optical module does not need any resin adhesive and fusible glass because a lens is press-fitted into position in the holder. The disclosed optical module is also advantageous in that the lens can quickly and easily be secured in position in the housing, the optical module is highly weather-resistant, highly reliable, and can be manufactured inexpensively.